1. Field of the Invention
The present invention relates to an electrode structure of an N-type III-V compound semiconductor element, and a method of manufacturing the same.
2. Related Background Art
A single element of Au, Ge or Ni, or an alloy thereof has been known as a low ohmic electrode material to be formed on the N-type III-V compound semiconductor. (Graham, and Steeds; Inst. Phys. Conf. Ser. No. 67: Section 10, p.507(1983), Kuan, Baston, Jackson, Rupprecht, and Wilkie; J.Appl. Phys. 54,6952 (1983)). In prior art electrodes which use those materials, a top surface of the electrode must be formed by an Au layer in order to allow press-contact of a bonding wire made of Au, and an annealing process is required to attain a low ohmic property.
However, during the annealing process, elements of the semiconductor, particularly a group III element diffuses to the top Au layer and separates on an electrode surface. As a result, the top Au layer is hardened and an oxide coating is formed. As a result, the wire bonding to the electrode is difficult to attain and the adhesion strength between the bonding wire and the electrode is materially lowered. In order to overcome the above problem, an intermediate layer (hereinafter referred to as a "stopper layer") for preventing the elements of the semiconductor from diffusing to the electrode surface is arranged between a semiconductor substrate and the top Au layer. By inserting the stopper layer, it is possible to suppress the diffusion of the elements of the semiconductor to the electrode surface while permitting the mutual diffusion at the vicinity of the interface of the electrode and the semiconductor.
An electrode structure which uses Mo has been known as the stopper layer for the low ohmic electrode structure of the N-type semiconductor element (JP-A-63-60526), but it has the following disadvantage. Usually, Mo is formed by thermal evaporation from a source heated by an electron beam, but because of a high melting point and a high thermal conductivity of this material, a temperature on a surface of a sample reaches above a heat resistance temperature (approximately 120.degree. C.) of a photo-resist during the evaporation. Thus, it is difficult to process the electrode to desired shape by a lift-off method which uses the photo-resist. Where the electrode is processed by etching, portions to be left are covered by the photo-resist and unnecessary portions are etched off by etchant or etching gas. However, no material which does not etch the photo-resist but etches only Mo has been reported.
The inventors of the present invention have prepared a low ohmic electrode structure by using Ti as the stopper layer (see FIG. 1) but it has been found that the single Ti layer does not function as the stopper layer. Detail thereof will be discussed hereinlater.